An aspect of the present invention features a method for manufacturing a substrate having a cavity. The method can comprises: (a) forming an upper layer circuit on an upper seed layer; (b) laminating a dry film on a portion of the-upper seed layer where a cavity is to be formed; (c) fabricating an upper outer layer by forming an insulation layer on top of the upper seed layer and on top and sides of the upper layer circuit; (d) stacking the upper outer layer on one side of a core layer where an internal circuit is formed; (e) removing the upper seed layer; and (f) forming the cavity by removing the dry film. The method for manufacturing a substrate with a cavity according to the present invention can reduce the total thickness of the substrate while the thickness of an insulation layer remains the same, by forming the insulation layer on sides of an external circuit.